Dawn, I think you're right about this with B12 and hyperthyroid symptoms. However, its extremely difficult to get any proof for it, as it doesn't show in blood tests (not for me at least) and I haven't seen anybody mentioning these symptoms with B12. The figures you mentioned are micrograms, not milligrams.
Therefore, be cautious at least with B12, don't use too much of it, it can mess up your thyroid for some days. I think this brain fog is caused by thyroid & adrenal problems, not only the adrenals. It's not helpful to say its caused by mercury or candida, one has to know it precisely in order to be able to address the problem. That said, I believe everyone who has a real mercury poisoning has some problems with the thyroid even if it appears normal. I see it so that it tries to readjust itself according to the current situation, any wins with chelation leaves it in a state that needs it to be redjusted, and you need to wait for it to readjust, before you continue to chelate. This advice goes only to those who hasn't got a major problem with thyroid that shows up in a blood test, I dont know about the rest.